I've run and, attended both Open Mics and Jam Nights and have tried to be the resident bassist on both types, stepping down when another Bassist turns up. Used to hate it when I'd turn up at someone else's Jam and see 5 other Bass cases in the pile and work out I'd get about 15 mins playing all night! There's one Blues jam I used to go to and the guy who ran it had his partner play Bass and no matter how many Bassists turned up you'd get your 15 minutes then she'd play the rest of the night. If you were on guitar or vox you'd get called back up, but never on Bass....
The worst one I ever did was Blues Jam which I ran until I totally lost patience with guitarists who spent 10 minutes setting out all their pedals , then tuning up ( having to borrow a tuner first-another 10 minutes) and then play over the other soloists ( sax and keys) at ear-splitting volume....